Youtube does this already. Are you using any kind of blocking programs?

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

[sigh] Take a look at the attached image, and you'll see what I mean by no left sidebar menu.

Someone could create a userscript to activate the button at the upper left (<button id="appbar-guide-button">). A user script is a lightweight piece of code you apply to sites using the Greasemonkey extension. To request a script be written, or to check whether it already exists, try these sites:

• OpenUserJS
  • Script search: https://openuserjs.org/?q=youtube
  • Request forum: https://openuserjs.org/corner

• GreasyFork
  • Script search: https://greasyfork.org/en/scripts/search?q=youtube
  • Request forum: https://greasyfork.org/en/forum/categories/script-requests

Note: As with any add-on, be cautious in selecting user scripts because they have significant access to your browser.

jscher2000 said

• • Request forum: https://greasyfork.org/en/forum/categories/script-requests

</blockquote>

Thanks. Request made (https://greasyfork.org/en/forum/discussion/6840/youtube-s-left-sidebar-menu).